A domestic steel sheet according to the related art was manufactured by post-processing a pre-coated metal (PCM) steel sheet or processing a steel sheet and then powder-painting the processed steel sheet. However, in accordance with the recent trend toward quality enhancement of electric home appliances, in the existing single colored PCM and powder-painting panel, implementation of a differential color and various glosses has been demanded. Therefore, a PCM laminating method using an ink printing has been used.
A laminate attached to the steel sheet by the PCM laminating method has an excellent gloss as compared to a paint painting system, and a separate ink printing process is not required. However, since the laminate is attached by only an adhesive, there are many problems such as poor adhesion with a material, deterioration of corrosion resistance, water resistance, and durability, and the like, as compared to the paint.
In addition, the laminate, which is a film having a thickness of 150 to 160 μm, has a thickness thicker than that of a coating-film of the paint, and a cost for quality enhancement may be excessively increased.
Further, since ink is already printed on a laminate film itself, it may be difficult to independently control a design in painting and electric home appliance companies. Further, at the time of fire, additional damages due to toxic gas poisoning or a large amount of smoke may be increased.
Among these problems, in order to satisfy the quality enhancement of a customer, painting using a 4-coating 3-baking system in which a painting equipment capable of performing the ink-printing on a painting line is provided and a transparent paint is painted by a wet-on-wet method has been attempted. In this ink-printing system, thin-film painting may be performed as compared to the laminate film, physical properties of a coating-film may be improved and economical efficiency may be obtained by a under coating paint-base paint-ink-transparent paint system. However, in the case of using to the existing resin in the above-mentioned method, it may be difficult to achieve low smoke density, a paint adhesion property, durability, and the like.
Meanwhile, an aliphatic polycarbonate resin is an eco-friendly material applied in various fields due to excellent processability and biodegradability. However, since the aliphatic polycarbonate resin has low elasticity and may be easily broken, there is a limitation in applying the aliphatic polycarbonate resin. Therefore, a technology of blending the aliphatic polycarbonate resin with various resins to improve heat resistance or mechanical properties has been required.
For example, a resin composition containing polypropylene carbonate melt-mixed with polymethyl methacrylate (PMMA) or a binder for a forming process of ceramics or metal powder has been disclosed in U.S. Pat. No. 4,946,884, but there is a limitation in improving physical properties only by blending polypropylene carbonate with a heterogeneous resin.
An interpenetrating polymer network (IPN) is a multi-component system polymer in which at least one component has a cross-linking structure and at least one component is polymerized or cross-linked in the presence of another component, such that polymer chains are interpenetrated into with each other. In the IPN, since a polymer chain degree is large due to the cross-linked structure, phase separation frequently generated in a polymer blend may be suppressed, and the IPN has a dual phase continuity structure in which two components form continuity, such that various physical properties may be improved.
Therefore, a technology of applying an IPN structure for applying a polypropylene carbonate resin to a PCM laminate film and overcoming limitation of the PCM laminate film to further increase an application range has been demanded.